The present invention relates generally to quality control in the production of optical fiber and the early detection of optical fiber defects. More particularly, the invention relates to a photodetection system and methods for the detection of flaws and surface defects, such as those caused by particles adhering to the surface of an optical fiber during its manufacture.
During the drawing of an optical fiber from a blank, various imperfections may occur. These imperfections include holes in the fiber, inclusions or particles within the fiber, particles on the surface of the fiber, and surface abrasions. The presence of surface abrasions and of particles on the fiber can cause the fiber to break at later stages of manufacture. Thus, quality control is critical during the manufacture of optical fiber both from the point of view of achieving the highest possible manufacturing yield. To this end, a variety of techniques for testing the quality of optical fibers are known. For example, tension screening of fiber may be performed off-line some time after manufacturing is completed. Defects or flaws result in fiber breakage. None of the presently existing techniques addresses the on-line real time detection of surface particles as optical fiber is drawn or manufactured.
The ability to detect particles on the optical fiber as it is drawn would be beneficial as both a process improvement tool and by providing the information necessary to reduce the number of particle induced breaks occurring later in the fiber optic manufacturing process. By way of example, real time detection of a large increase in the number of flaws might indicate that the furnace refractories are deteriorating and the furnace should be replaced.
The present invention recognizes that a fiber optic perform and a drawn fiber including the cladding prior to its coating form excellent waveguides. As such, light permeates substantially the entire cross section of the fiber prior to coating. The majority of the light is guided down the entire length of the fiber unless scattered out along the length of the fiber by an imperfection, such as a hole, surface abrasion, or particle. During the drawing process, optical fiber is filled with light in the draw furnace which is an excellent source of intense white light. As the fiber is drawn, light from the draw furnace is conducted along it and scattered out as a result of imperfections. Because of the speed at which the fiber is moving as it is being drawn, the scattered light will be seen by a detector as a brief flash as the defect rapidly passes the detector.
Among the other needs addressed by the present invention is the need for a detector which can detect flashes caused by scattering of light due to imperfections in an optical fiber as the fiber is being drawn. The present invention also provides a detector which can operate effectively at the normal speed at which optical fiber is drawn to identify a flash caused by the presence of a particle on the surface of the optical fiber. Such particles may be referred to herein as particles on glass or POGs.
One aspect of the present invention preferably comprises a high speed large area detector placed at one focal point of an elliptical mirror. The optical fiber to be subjected to detection is placed at the other focal point of the mirror. The mirror arrangement provides that 180 degrees of the light scattered from the fiber will be reflected onto the detector. The detection components are preferably shielded to block out stray light and purged with an inert gas to keep their optical surfaces clean.
According to another aspect of the invention, an elliptical mirror is used. The fiber is placed at one focal point of the mirror and a fluorescent rod which preferably has a diameter of 1 cm is placed at the other focal point. The rod preferably has polished ends and is oriented parallel to the fiber. One end of the rod is preferably mirrored, while a high speed Silicon detector is attached to the other end. The rod is doped with a fluorescent dye which absorbs visible light, preferably yellow light, and fluoresces in the visible or near IR part of the spectrum. As the light from the fiber hits the rod, fluorescence will occur. Approximately half of the light will fluoresce out of the rod and be lost. One quarter of the light will be guided inside the rod, reflect off the mirrored surface and then be guided back down to the detector, while the remaining quarter of the light will be guided directly to the detector. Fluorescence occurs very fast, on the order of 10 ns, so even very fast flashes may be readily detected. Also, since the rod has a diameter of 1 cm, all of the light scattered from the fiber will hit the rod and the system will be insensitive to fiber movement. Further, it will be recognized that by increasing the length of the rod the time during which a flash will be observed will be increased. Additionally, the rod may be preferentially masked to distinguish between particles and holes, for example.
Another aspect of the present invention preferably comprises a two-view system. Each view includes a small area, high-speed, sensitive Silicon detector, two lenses, and a spherical mirror. The lens system makes the view less sensitive to fiber movement. In the preferred embodiment of the invention, lenses are used which allow the system to tolerate fiber movement of 2.5 mm.
The lenses are placed on one side of the fiber, and the spherical mirror is placed on the other side of the fiber at its radius of curvature away from the fiber. All the incident light from the fiber is thus reflected back to the detector. Using two views spaced 90 degrees apart increases the likelihood that a flash from a POG will be collected. Also, it is believed that signal differences between the detectors may be used to discriminate particle type and hole presence given the addition of suitable signal processing support to rapidly analyze the signal differences. The system is preferably housed in a box to block out stray light and purged with an inert gas. The housing will preferably be mounted on adjustable x and y stages to properly locate the detector with respect to the fiber. A source of purge gas will be supplied to keep the optical surfaces of the detector contamination free.
Another aspect of the invention preferably comprises a three-view system. Each view includes a small area, high-speed, sensitive Silicon detector, three lenses, and a spherical mirror. The lenses are placed on one side of the fiber, and the spherical mirror is placed on the other side of the fiber at its radius of curvature away from the fiber. All the incident light from the fiber is thus reflected back to the detector. Using three views spaced 120 degrees apart eliminates any dead zone in which the fiber can be present without a flash being detected. The use of three views and three lenses permits a detector according to this aspect of the invention to detect flashes at high speeds, such as typical draw speeds used during optical fiber manufacturing, given the proper selection and configuration of parts. Further, while a variety of approaches are described in detail, it will be recognized that multiple view systems may be employed with the number of views determined by the application and considerations such as cost. Various other optical arrangements and detectors may be suitably employed.
A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following Detailed Description and the accompanying drawings.